Compositions: ceramic – Ceramic compositions – Devitrified glass-ceramics
Reexamination Certificate
2002-10-19
2004-11-16
Stein, Stephen (Department: 1775)
Compositions: ceramic
Ceramic compositions
Devitrified glass-ceramics
C433S201100, C433S212100, C106S035000
Reexamination Certificate
active
06818573
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the molding of metal free dental restorations such as crowns, bridges, inlays, onlays, etc., from glass-ceramic materials. The invention is also concerned with apparatus which permits dental restorations to be easily molded from glass-ceramic materials and, lastly, the invention relates to the resulting dental restorations.
BACKGROUND OF THE INVENTION
In the prior art, various methods have been used to form metal free dental restorations from glass-ceramic materials. Glass ceramic materials because of their strength, translucency, non-toxicity and other physical properties are ideal materials for use in forming dental restorations. Because of their suitability, glass-ceramic materials have been used to form dental restorations for at least eighty years. In the prior art, the most widely used means for forming dental restorations from mixtures of glass and ceramic materials is a process which utilized a slurry of glass and ceramic particles. In this process, a die is formed which is an exact replica of the remaining portion of the tooth to which the restoration is to be secured. It is understood by one skilled in the art that the remaining portion of the tooth has been prepared by the dentist in such a manner that the attachment of the restoration is facilitated. To start the process, the dentist takes an impression of the prepared tooth to create a negative impression of the prepared tooth or teeth. This negative impression is then packed with a material to form a positive impression of the prepared tooth or teeth. This positive impression is called a die. Platinum foil is then pressed over the die to form a matrix which is essentially a foundation on which the dental restoration is built. In order to form up the dental restoration, many layers of a slurry of a particulate glass-ceramic material are applied to the platinum foil matrix. As multiple layers of the slurry are built-up and dried, a semi-solid structure is formed which can be carved into shape of the desired dental restoration. Once the desired shape is achieved, the structure is then removed from the die. At this stage, the structure is referred to as a green structure. The structure is then fired. During the firing process, the particulate glass-ceramic material fuses into a solid mass. Because the green structure is formed from multiple layers of the dried glass-ceramic material, uneven fusing may result. As a result of this uneven fusing, the physical properties of the finished restoration may be detrimentally affected, thereby creating an inferior dental restoration. Further, as can be seen from the above description, the overall process is very labor intensive.
Other methods for forming the green restoration have been considered in the prior art, for example, in U.S. Pat. No. 2,196,258, a mixture of particulate glass and ceramic materials which incorporates a binder is packed into a flexible mold to form a green structure which is then fired to form a finished structure. Again, because the process entails the fusing of particulate material, uneven fusing may result and, hence, a weakened and inferior dental restoration may result.
To overcome the problems as described above, the trade has recognized that in order to produce strong, translucent metal free dental restorations, it would be desirable to form these restorations directly from a homogenous, molten glass-ceramic material. It was realized that it may be possible to produce a satisfactory restoration by forcing a molten or plastic glass-ceramic material into a mold having a cavity in the form of the desired dental restoration. The prior art further recognized that the glass-ceramic material could be introduced into the cavity when the glass-ceramic material was in the liquid or in the plastic state.
A constant goal of the prior art as described above was to effect the molding process in a quick and efficient manner, and in a manner that produces a dental restoration that has excellent definition and fit. In dental restorations, definition is extremely important. In order to have a satisfactory restoration, the finest details of the original tooth must be reproduced. For example, for a dental restoration to be successful, the margins must be sharp and well defined. It is in this area that the prior art molding processes are deficient, in that it was not possible to achieve the desired degree of definition.
Further, it is desirable to produce a dental restoration in a short period of time in order to efficiently utilize the overhead of the dental laboratory, and in order to minimize the labor content of the dental restoration.
Dental laboratories are not typically well funded operations. Therefore, in order to keep costs to a minimum, it is highly desirable that a suitable process for forming dental restorations utilize equipment which is relatively inexpensive. While the above described process fits this requirement, the below described DI-COR process does not.
As is discussed above, there are several prior art processes for the manufacture of dental restoration from glass-ceramic materials. A recent addition to the prior art is the DI-COR process as sold by Dentsply International, Inc., of York, Pa. In this process, a dental restoration is formed by centrifugal casting of a molten glass-ceramic material. This process is further described in U.S. Pat. No. 4,431,420, issued Feb. 14, 1984, and related patents. Centrifugal casting has been extensively used in the casting of metals principally by the lost wax process. Further, this process has been imminently successful for hundreds of years for use in conjunction with metals. This success results from the fact that molten metals have very low viscosity and high density in the molten state, hence, they function very well in centrifugal casting processes. That is, because molten metals have a high density and a very low viscosity in the molten state, centrifugal force is adequate for purposes of injecting the molten metal into a preformed mold cavity. In an attempt to produce dental restorations which have high definition, the above-mentioned DI-COR process uses centrifugal force to form the desired dental restorations from a molten glass-ceramic material. Molten glass-ceramic materials have a much higher viscosity, and a much lower density when compared to molten metals. For this reason, it is not possible to consistently drive a molten glass-ceramic material by centrifugal force alone into a mold in order to produce a satisfactory dental restoration. That is, a molten glass-ceramic material cannot be driven by centrifugal force into a mold cavity with sufficient force in order to always get the required definition, which is necessary to form a satisfactory dental restoration. It is well recognized by one skilled in the art, that in order to have a satisfactory dental restoration, excellent definition must be achieved in order to recreate the desired margins, which are needed for the proper fit of a dental restoration into the human mouth.
Further, the DI-COR process is deficient as to the coloration of the glass-ceramic material utilized. The resulting DI-COR dental restoration had an undesirable white color and must be glazed in order to produce satisfactory human coloration. As a result, the coloration is only on the surface of the dental restoration. If adjustment by grinding is needed in the final installation of the restoration into the human mouth, the glazing is removed, thereby exposing the whitish base which contrasts with the glaze. This contrast is very unsatisfactory from an esthetic point of view.
In contrast to this deficiency, the restoration of the subject process is adapted to utilize glass-ceramic materials, wherein the coloration of the resulting dental restoration throughout approximates human tooth coloration. Hence, if grinding is necessary in final fitting, contrast between the surface of the dental restoration and the underlying base is not observed.
Undesirable contrast can also result from normal wear, wherein a
Bahr Donald R.
Stein Stephen
TEC Ventures, Inc.
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